Idiopathic Respiratory Distress Syndrome (IRDS) remains the major cause of neonatal death in the United States. Many biochemical assay techniques have been developed using amniotic fluid collected from the gravid uterus to indicate whether RDS would develop if delivery occurred immediately after measurement. Several of these biochemical assay techniques have excellent sensitivity and specificity for determining the occurrence of RDS postpartum, but their value for prediction of future dates of maturation or potential to respond to glucocorticoid treatment is quite limited. In addition, amniocentesis, central to all the assay techniques, is an invasive procedure of moderate risk. We propose to examine the use of noninvasive diagnostic ultrasound as an in utero, predictive test of the occurrence of RDS following delivery. Preliminary work on fetal sheep under ideal experimental conditions indicates a strong correlation between quantitative and visual analysis of backscattered ultrasound and the fetal lung's morphological and functional maturity. To demonstrate the applicability of the technique in the real world of the clinic we will modify the methods of the preliminary studies. We will acquire B-mode echograms both on multiformat films for visual analysis and in video format for quantitative analysis of the ratio of lung to liver mean backscattered amplitude. In addition, backscattered radio frequency (rf) ultrasound will be acquired in digital format. Computer processing of the digital data will yield quantitative estimations of fetal organ attenuation and backscatterer size distribution that are insensitive to overlying tissues or scanner TGC settings. Ultrasonic fetal lung measures from both visual and quantitative analysis will be correlated to morphological and functional lung maturity in neonates.